Ultrashort Power-Dips in Fourier Domain Mode-Locked Lasers: Impact of Picosecond Carrier Recovery

Fourier domain mode-locked (FDML) lasers are widely used in applications requiring high-speed wavelength sweeps and reliable spectral stability, such as optical coherence tomography (OCT) [1]. In this study, we explore the steady-state behavior of FDML lasers when the carrier lifetime of the semiconductor optical amplifier (SOA) is reduced to one picosecond-a scenario that can enable reduced intensity noise, improved coherence, and higher sweep speed, achievable with advanced quantum-well or quantum-dot SOAs, opening possibilities for next-generation FDML lasers [2], [3]. In previous studies, SOA carrier lifetimes longer than 70 picoseconds yielded irregular dips (holes) with varying shape, amplitude, and duration in the output power pattern, hindering beam coherence except under ultra-stable conditions [1,3-5]. Our latest simulations, which align with experimental findings in the detection and profiling of such dips [3]–[5], reveal that a 1 ps carrier lifetime improves stability and coherence but leads to emergence of consistent ultrashort, sinc-like power dips (Fig. 1, middle) for high output powers, which are almost uniform in duration (Fig. 1, right). The density of these dips increases as the output power is raised toward the upper practical limit. Based on foundational FDML laser theory [5]–[6], Equations (1)-(3) explain the formation of these ultrafast dips. At high photon flux, rapid gain depletion causes a sharp drop in carrier density $(N)$, generating a dip. Given the ultrashort carrier lifetime $(c)$, the carriers recover quickly after depletion, restoring gain for the next dip. The time-delayed feedback term in Equation (2) represents light from previous round trips interacting with the restored carriers, amplifying the dips.